1. Technical Field
The disclosure is related to a pixel driving circuit of an organic light emitting diode, and more particularly, to a pixel driving circuit of an organic light emitting diode that is capable of compensating a threshold voltage of a transistor.
2. Related Art
FIG. 1 is a diagram of a conventional organic light emitting diode (OLED) display panel. The display panel 10 includes a data driver 11, a scan driver 12 and a display array 13. The data driver 11 controls data lines DL1-DLn, and the scan driver 12 controls scan lines SL1-SLm. The display array 13 includes a plurality of pixel units, each disposed at corresponding intersections of the data lines DL1-DLn and the scan lines SL1-SLm. For example, the display unit 14 is disposed at the intersection of the data line DL1 and the scan line SL1. As illustrated in FIG. 1, the equivalent circuit of the display unit 14 (and also those of other display units) includes a switch transistor T11, a storage capacitor C11, a driving transistor T12 and an OLED D11, where the switch transistor T11 and the driving transistor T12 are N-type transistors.
The scan driver 12 sequentially outputs scan signals to the scan lines SL1-SLm so that the switch transistors in the display units coupled to a certain row are turned on at the same time, while switch transistors in the display units coupled to all other rows remain turned off. According to image data to be displayed, the data driver 11 outputs corresponding video signals (gray levels) to display units of one row via the data lines DL1-DLn. For example, when the scan driver 12 outputs scan signals to the scan line SL1, the switch transistor T11 of the display unit 14 is turned on. The data driver 11 outputs the corresponding pixel data to the display unit 14 via the data line DL1, thereby storing the pixel data voltage in the storage capacitor C11. The driving transistor T12 then provides driving current Ids to drive the OLED D11 according to the voltage stored in the storage capacitor C11.
Being a current driven component, the luminescence of the OLED D11 is determined by the value of the driving current Ids. The driving current Ids is the current flowing through the driving transistor T12, which may be represented by formula (1):Ids=½k(Vgs−Vth)2  (1)
where k represents the conduction parameter of the driving transistor T12, Vgs represents the voltage difference between the source and the gate of the driving transistor T12, and Vth represents the threshold voltage of the driving transistor T12.
However, due to process factors of a thin-film transistor, transistors in different regions of the display array 13 possess varying electrical characteristics, meaning the transistors possess different threshold voltages. Hence, when transistors in different regions receive pixel data of the same voltage, the threshold voltage variation of the transistors causes the driving currents provided to the OLEDs to be inconsistent, consequently causing the OLEDs to generate different luminescence when receiving pixel data of the same voltage, and inconsistent luminance results throughout the image displayed by the display panel 10.